1. Field of the Invention
The present invention relates to a retro focus type lens having a short entire length which is suitable for an image pickup device such as a video camera or a digital still camera.
2. Related Background Art
In recent years, various video cameras and various digital still cameras, each having a solid-state image pickup element such as a CCD sensor or a CMOS sensor have been developed. In such kinds of image pickup devices, various glass materials such as a low pass filter and a color filter are disposed between the trailing end of the photographing lens and the image pickup element in many cases. Thus, as compared with a focal length, it is necessary to lengthen a back focal distance.
As a type of a lens having a long back focal distance, a retro focus type lens has been known up to now. For example, a retro focus type lens composed of five lenses, that is, a first lens having a negative meniscus shape whose convex surface faces an object side, a second biconvex lens, a third biconcave lens, a fourth lens having positive refracting power, and a fifth biconvex lens, which are disposed in order from the object side is disclosed in JP 46-024194 B. According to a numerical embodiment in this document, the back focal distance becomes about 1.3 times than that of the focal length.
Also, in recent years, according to the trend toward increasing the number of pixels of the solid-state image pickup element, the performance required for the photographing lens has been extremely increasing. For example, JP 63-081414 A, JP 03-063613 A, JP 10-213742 A, JP 10-293246 A, JP 2001-100091 A, and the like disclose retro focus type lenses in which a high imaging performance and a long back focal distance can be attained together, and the lenses are composed of five lenses, that is, a negative lens, a positive lens, a negative lens, a positive lens, and a positive lens.
Here, in the case of the structure of, the five lenses, that is, the negative lens, the positive lens, the negative lens, the positive lens, and the positive lens, when an interval between the first lens and the second lens is narrowed, it is difficult to obtain a sufficient back focal distance. Thus, if the back focal distance is forcedly lengthened, the power of the first lens becomes stronger, so that aberration correction becomes difficult and the sensitivity of performance deterioration resulting to parallel and tilt eccentricities of the first lens becomes higher. Conversely, if the interval is widened, the outside diameter of the first lens is made large and the back focal distance is made too long. Thus, a problem in that a size of the entire lens is increased is caused.
According to JP 63-081414 A and JP 10-293246 A, the interval between the first lens and the second lens is relatively narrow, so that it is advantageous to reduce the size of the lens. However, aberration correction is difficult.
Also, as to an interval between the second lens and the third lens, when the interval is narrowed, it is difficult to dispose a stop between the second lens and the third lens. Thus, a sufficient exit pupil distance cannot be obtained. Conversely, if the interval is widened, a lens diameter of the entire lens is made large and a height in which an off-axis light flux is passed becomes relatively higher. As a result, a problem in that the correction of off-axis aberration becomes difficult is caused.
According to JP 10-213742 A and JP 2001-100091 A, the interval sufficient to dispose only the stop is kept between the second lens and the third lens. However, when it is considered that a mechanical shutter unit is further disposed, it cannot be said that the sufficient interval is kept. In addition, in the case of the video camera or the digital still camera, it is necessary to lengthen the back focal distance. However, the back focal distance longer than is necessary is not preferable because the entire length of the photographing lens becomes longer.
According to JP 03-063613 A, the back focal distance is equal to or more than 1.2 times as long as the focal length. Thus, there is a problem with respect to the entire length of the photographing lens. In addition, a retro focus type lens which is composed of five lenses, that is, the negative lens, the positive lens, the negative lens, the positive lens, and the positive lens and in which further performance improvement is achieved using an aspheric surface has been proposed in, for example, JP 09-166748 A.
According to JP 09-166748 A, one surface of the fifth lens disposed in the nearest side of an image is formed to be an aspheric surface, thereby satisfactorily correcting particularly various off-axis aberrations.
An object of the present invention is to provide a lens system in which the above-mentioned conventional defects are solved, and a sufficient back focal distance is kept, and which is small in size, low in cost, and satisfactory in an optical performance.
In order to achieve the above-mentioned object, a lens system according to one aspect of the present invention includes a first lens of negative optical power (reciprocal of focal length) having a meniscus shape whose convex surface faces the front, a second lens of positive optical power forming a biconvex shape, an aperture stop, a third lens of negative optical power forming a biconvex shape, a fourth lens of positive optical power, and a fifth lens of positive optical power forming a biconvex shape, which are disposed in order from the front (object side in the case of the photographing lens) to the rear (image side in the case of the photographing lens), the lens system being characterized in that the following conditions are satisfied.
0.5 less than DB/f less than 0.8xe2x80x83xe2x80x83(1),
nd2 less than 1.7xe2x80x83xe2x80x83(2),
and
0.8 less than bf/f less than 1.0xe2x80x83xe2x80x83(3),
where
DB denotes an interval between the second lens and the third lens,
nd2 denotes a refractive index of a material composing the second lens,
bf denotes a back focal distance, and
f denotes a focal length of the entire system.
In further another aspect of the lens system, it is preferable that the following conditions are further satisfied.
0.6 less than DA/f less than 1.0xe2x80x83xe2x80x83(4),
0.45 less than fA/fB less than 0.7xe2x80x83xe2x80x83(5), and
1.4 less than f5/f4 less than 2.0xe2x80x83xe2x80x83(6),
where
DA denotes an interval between the first lens and the second lens,
fA denotes a total focal length from the first lens to the second lens,
fB denotes a total focal length from the third lens to the fifth lens,
f4 denotes a focal length of the fourth lens, and
f5 denotes a focal length of the fifth lens.